edit-1.diff

+type ClearAction = {type: 'CLEAR'};
+type ResetAction = {type: 'RESET', text: 'Hello World'};
+type UpdateAction = {type: 'UPDATE', text: string};
+
+type Action =
+  | ClearAction
+  | ResetAction
+  | UpdateAction;
+
 const CLEAR = 'CLEAR';
 const RESET = 'RESET';
 const UPDATE = 'UPDATE';
 
-function reducer(state: string = '', action) {
+function reducer(state: string = '', action: Action) {
   switch (action.type) {
     case CLEAR:
       return '';

reducer.2.js

type ClearAction = {type: 'CLEAR'};
type ResetAction = {type: 'RESET', text: 'Hello World'};
type UpdateAction = {type: 'UPDATE', text: string};

type Action =
  | ClearAction
  | ResetAction
  | UpdateAction;

const CLEAR = 'CLEAR';
const RESET = 'RESET';
const UPDATE = 'UPDATE';

function reducer(state: string = '', action: Action) {
  switch (action.type) {
    case CLEAR:
      return '';

    case RESET:
      return action.text;

    case UPDATE:
      return action.text;

    default:
      return state;
  }
}

function clear() {
  return { type: CLEAR };
}

function reset() {
  return { type: RESET, text: 'Hello World' };
}

function update(text: string) {
  return { type: UPDATE, text };
}

reducer.js

const CLEAR = 'CLEAR';
const RESET = 'RESET';
const UPDATE = 'UPDATE';

function reducer(state: string = '', action) {
  switch (action.type) {
    case CLEAR:
      return '';

    case RESET:
      return action.text;

    case UPDATE:
      return action.text;

    default:
      return state;
  }
}

function clear() {
  return { type: CLEAR };
}

function reset() {
  return { type: RESET, text: 'Hello World' };
}

function update(text: string) {
  return { type: UPDATE, text };
}